Hulu seems to be finally caving into the pressures of making money that are inevitably placed on free services that grow quite large. Currently, Hulu is allowed to have no more than 5 episodes from any series available at one time. Meaning that in order to add new content the old content must go and this is where the $10 a month subscription comes in. Hulu plus subscribers will be offered a more comprehensive collection to choose from but how extensive is still uncertain. 10$ isn't a lot but it might not get you much either. Hulu has to pay $1 to $1.50 per subscriber to each of the networks with content on the site for retransmission purposes as well as streaming costs. In the end Hulu will need to pay $3 to $5.50 for every $10 subscription.
Another issue is that Hulu needs to get on other platforms like the mobile platforms, Roku, and the iPad,for which there could be an app currently in development. Eventually Hulu would want to make itself into a Netflix of sorts where you can stream video instantly and not deal with the buffering issues Hulu has even if your internet connection is fast. Right now the content offered by Hulu is limited to ABC, NBC and Fox so if they do want to build Hulu Plus in the future they will have to find a way to expand content and up their prices without turning subscribers away. The further Hulu Plus deviates from the original Hulu model of being the "catch-up website" the better chance it has at succeeding.